Nitride semiconductors, which have high breakdown electric field and high saturation electron velocity, have been attracting attention as the next-generation semiconductor materials for high-frequency/high-power devices. In particular, a multi-layer structure, which is formed by laminating a layer composed of AlGaN and a layer composed of GaN, produces a high two-dimensional electron gas (2DEG) at a lamination interface (hetero interface) owing to large polarization effects (spontaneous polarization effect and piezo polarization effect) inherent in nitride materials, and thus, high electron mobility transistors (HEMTs) including such a multi-layer structure as a substrate have been vigorously developed (for example, see Non-Patent Document 1).
HEMTs, which are operated under the conditions of high power and high frequency (100 W or more, 2 GHz or more) such as ones for mobile phone base stations, are desirably produced using materials having heat resistant as low as possible to limit a temperature rise of a device due to heating. Contrastingly, HEMTs, which perform a high-frequency operation, are desirably produced using highly insulating materials because they need to reduce parasitic capacitance as much as possible. In the production of a device that satisfies the above-mentioned requirements using a nitride semiconductor, a semi-insulating SiC substrate having a resistivity as high as 1×108 Ω·cm or more is used as a base substrate because such a substrate allows for the deposition of a good nitride film.
It is proposed to deposit an insulating AlN film on a conductive SiC substrate by the method such as the hydride vapor phase epitaxy method (HVPE method) or the MOCVD method and use it as a base substrate (for example, see Non-Patent Document 2).
In the technique disclosed in Non-Patent Document 2, however, since the crystal quality of a nitride epitaxial film formed on the base substrate depends on the quality of the AlN film formed by the HVPE method, the quality of the AlN film is required to be improved for improved quality of the nitride epitaxial film. Unfortunately, it is difficult to control the deposition of the AlN film by the HVPE method in such a way that the crystal quality (such as dislocation density) becomes uniform over the entire wafer in the deposition, leading to inplane variations in characteristics of an epitaxial film, further, of a device.
The approach capable of achieving effects similar to those in the case of using a semi-insulating SiC substrate with the use of a base substrate including a vanadium-doped semi-insulating SiC film formed on a conductive SiC substrate has been known (for example, see Patent Document 1).
In recent years, gallium nitride (GaN) substrates expected to have improved performance and reliability have been in practical use as the base substrate for HEMT device. The approach of manufacturing a GaN substrate by the gas phase process or liquid phase process has been known (for example, see Patent Documents 2 and 3).
As described above, in use of a nitride semiconductor for high-frequency application, it is desirable that the substrate be free from parasitic capacitance. Thus, a semi-insulating GaN substrate is desirably used even in the use of a GaN substrate, but now, a semi-insulating GaN substrate is expensive and is hard to obtain. In contrast, a conductive gallium nitride substrate is relatively inexpensive and is easy to obtain because conductive gallium nitride substrates are in mass production for vertical LDs.
An approach of forming a carbon (C) doped GaN layer on a conductive GaN substrate to obtain a GaN substrate that can be used for high-frequency applications, in which the above-mentioned problem is taken into consideration, has been known (for example, see Patent Document 4). In the technique disclosed in Patent Document 4, however, the C concentration of an electron transit layer becomes higher, which makes it difficult to improve device performance.
There is a known technique of doping zinc (Zn) to obtain a high-resistance nitride single crystal (for example, see Patent Document 5).